Commonize PCIe pins, as the configuration is SoC-common
and doesn't change (or at least doesn't change much) between
boards.

While at it, remove "output-low" from the RB5 board, as it's
not necessary - we already explicitly pull the perst pin low.
Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Link: https://lore.kernel.org/r/20210616122708.144770-2-konrad.dybcio@somainline.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>